1. Field of the Invention
This invention relates to a method for producing a pasted SOI substrate and to an SOI substrate and more particularly to a method for producing of a pasted SOI substrate which has repressed voids liable to occur particularly in the most peripheral part thereof, widened the area thereof available for the manufacture of devices, increased the number of devices derivable therefrom, and improved the yield and quality of the SOI substrate.
2. Related Background Art
The formation of a single crystal Si semiconductor layer on an insulator is widely known as the "silicon on insulator (SOI)" technique. Many studies have been made on this substrate because the substrate possesses numerous excellent characteristics which are not exhibited by standard bulk Si substrates for the manufacture of Si integrated circuits.
In recently reported SOI structures, the so-called "pasted SOI" stands out particularly in terms of quality. This pasted SOI is the product of a technique which comprises preparing two wafers at least either of which has an insulating film formed by oxidation, tightly joining the two wafers on the mirror faces thereof, heat-treating the joined wafers thereby fortifying the bond at the tight interface, and then polishing or etching either of the substrates of the joined wafers until a Si single crystal thin film of an arbitrary thickness remains on the insulating film.
Many studies have been made concerning the mechanism of the pasting which constitutes the backbone of this technique. The method for cleaning the two substrates before they are pasted to each other is considered one of the important issues in these studies. Scientists have ascertained that the two substrates are first given weak bondage by the Van der Waals force during their contact at room temperature and then given strong interatomic bondage by a subsequent heat-treatment at an elevated temperature. The strength of this Van der Waals bondage varies widely depending on the method used for cleaning the substrates. In Proceedings of 4th International Symposium on Silicon-on-Insulator Technology and Devices, May 6-11, 1990, Montreal, for example, Abe et al., regarding the pasting of Si and SiO.sub.2, report "that while the wafers cleaned with a HF solution are bonded only with great difficulty, the wafers which have undergone the so-called "RCA cleaning" are bonded easily because they have formed a spontaneous oxide film on their surfaces". Many other researchers assert that the water adsorbed to the interface of the pasted substrates plays an important role in the bondage by the Van der Waals force and, therefore, the silicon surfaces are required to be hydrophilic. In Japanese Journal of Applied Physics, vol. 30, no. 4, April 1991, pp. 615, Mitani et al. report "that the wafer surfaces cleaned by the HF dipping generate more bubbles in the range of annealing temperatures of 200.degree. C.-800.degree. C. than the wafer surfaces cleaned by the dipping in NH.sub.4 OH+H.sub.2 O.sub.2, notwithstanding the former wafer surfaces are inferred to have less --OH groups than the latter wafer surfaces".
It has been consequently held generally that when Si and SiO.sub.2 are pasted, perfect bonding cannot be obtained unless at least the Si side has undergone a treatment to impart hydrophilicity.
The present inventors, in their experiment which comprises pasting a Si wafer treated to acquire hydrophilicity and a Si wafer provided with an oxide film and heat-treating the pasted wafers, have found that voids considered to have originated in bubbles occur at a high density in the peripheral part of the substrate (in an annular area within about 3 mm-4 mm from the wafer edge) (hereinafter referred to as "peripheral voids"). The group of voids leads to a possible decrease in the area available for the formation of devices and, at the same time, may cause exfoliated fragments of films to adhere to the water surfaces, thereby hindering the device manufacturing process.